1. Field Of The Invention
This invention relates generally to gas bearings as used, for example, in free-piston Stirling machines, and relates more particularly to a restrictor apparatus used with gas bearings.
2. Description of the Related Art
In many different machines, pistons reciprocate in a cylinder formed in a housing. Due to accurate machining, a thin annular gap is formed between the cylinder wall and the piston wall. In Stirling cycle machines, for example, the housing encloses a work space bounded by one end of the piston and a back space bounded by the opposite end of the piston. The term “piston” can refer generically to any piston-like body, including the displacer in a Stirling cycle machine. A working gas, such as helium, fills the workspace, back space and other regions of the machine within the housing.
Because of the close proximity of the piston and cylinder walls during operation, the annular gap formed between the walls must be lubricated to prevent rapid wear. The most effective lubrication has been found to be a thin layer of the working gas forming a gas bearing. Such gas bearings are described in U.S. Pat. Nos. 4,412,418, 4,802,332 and 4,888,950, all to Beale, which are incorporated by reference.
In order to lubricate the moving piston, gas must be directed into the gap at three or more points around the peripheral surface of the piston after being routed from the workspace or back space. Transporting the gas into the annular gap often requires a network of small passages. The passages that route the working gas directly into the annular gap are often extremely small to restrict the flow of gas. Restricting the flow of gas is necessary to maintain a constant gas pressure, but very small passages and other structures that restrict the flow of gas into a clearance gap, commonly referred to as “restrictors”, are especially susceptible to blockage.
Conventionally, gas bearing restrictors have been provided by a number of means: capillary tubes, screws and close fitting parts with accurate passages used to direct the gas into the clearance gap. All of these previous techniques and structures suffer from cost or sensitivity to blockage by small particles in the working gas. A desirable restrictor would have low cost, temperature and creep stability and little or no susceptibility to blockage.